1. Field of Invention
This invention generally relates to coating materials and to a process for preparing antifouling coating compositions incorporating metal oxides, particularly cuprous oxide in water-based paints, which are suitable for use in protecting underwater surfaces from hard fouling and slime build up.
2. Description of the Prior Art
In the prior art metal oxides have been used as toxicant pigments dispersed in solvent-based film forming vehicles. The most widely used form of such prior art is the red vinyl cuprous oxide antifouling paint, known as paint Formula No. 121, which consists of as much as 70-72% by weight of cuprous oxide with 86% copper content. The black version of this paint, known as paint Formula No. 129, consists of 58% cuprous oxide with 13% black iron oxide representing 68.5% copper in the pigmentation. Plastic shipbottom paints may contain a combination of 40% cuprous oxide with 11% zinc oxide and 3.8% magnesium silicate extender. For underwater use on rubber surfaces, a polyisobutylene polymer and 52.8% by weight of cuprous oxide or mercury compounds including mercuric oxide as toxicants are employed in antifouling paint coatings. Other heavy metal toxicant agents, including tin, lead, and arsenic compounds, and other organic toxicants, including fungicides and biocides, are also employed in antifouling paints that use cuprous oxide as the primary toxicant.
All of the above formulations are organic solvent-based paints and therefore release, on application, undesirable solvent vapors into the atmosphere. They also release afterwards, on water immersion, retained organic solvent matter into the water of the environment. While the normal mechanism of toxicant release involves a gradual loss of the soluble vehicle components by lixiviation, subsequent loss of the cuprous oxide is experienced by chemical reaction with seawater forming copper complexes of varying solubilities, depending upon the local concentration of hydrogen ion. Furthermore, the cuprous oxide can react during initial dispersion with the film forming polymeric resinous components of the vehicle, in particular the abietic acid of the rosin, so as to form copper compounds, such as copper resinate, which are also soluble and which may result in considerable amounts of toxic underwater releases.
The process of this invention is different from the present emulsification treatment used to incorporate inorganic metal oxides in some water-based latex paint formulations. Metal oxides have been emulsified for dispersion in water-based paints by reacting them with lecithin, a naturally occuring compound that consists of a mixture of diglyceride esters of certain fatty acids linked to the choline ester of phosphoric acid. These emulsified metal oxides, however, do not effectively release the metal toxicant when incorporated in a water-based latex formulation. The fluid lecithins are soluble in organic solvents only, and do not represent the products of polymerization treatment. Thus, even when the so reacted metal oxides are recovered and are again dispersed in water-based paints, they do not release toxicant effects themselves and are useful in antifouling paints only when organometal toxicants have been added.
Water-based antifouling paint formulations based on latex resins have not been used commercially for fouling protection in marine immersion applications. The previous water-based antifoulant paint formulations utilize water-based polymer latexes or water-dispersed alkyd resins with inorganic pigments which are not, or only slightly, reactive with such water-dispersed binder vehicles. They therefore, did not interfere with the stability and application of the water-based paints but, on the other hand, they did not contribute to the antifouling protection. This aspect was left to be accomplished by the incorporated organometal toxicant. Heavy metal inorganic antifoulants such as cuprous oxide have not previously been used due to the reaction with the vehicle. The reaction product of the metal oxides with the latex resin forms clumpy or grainy reaction products which settle during storage and are difficult to redisperse thus deteriorating paint application and performance.
Although the organometal toxicants are compatible with latex based antifouling formulations, most coating systems of this type contain various water soluble pigments, fillers and binders so that the organometals diffuse into the immersion water at high initial rates which decrease logarithmically with time. Various polymeric binder compounds have been developed to control the release rates of these organometal toxicants as exemplified by, for example, U.S. Pat. Nos. 3,016,369; 3,382,264; 3,930,971; 3,979,354; 4,064,338; 4,075,319; 4,174,339; 4,389,460; and 4,480,056.